I decided to explore this a bit more.
I have two computers: an older Lenovo T520 laptop running Windows 7 Pro (Intel i7 CPU), and a new (as of November) Lenovo desktop workstation (Xeon CPU) running Windows 10 Pro for Workstations version 20H2, OS build 19042.985.
When I first installed MusicBee on the new Win10 machine I installed v3.3.7367 and I used exported settings from the laptop install of MusicBee, so both have the same settings, skin, etc. I later updated both the machines to v3.4.7805. So both machines now have identical MusicBees but only the Win10 machine has the volume slider behavior.
As an experiment I put the portable version of MusicBee in a separate partition on both computers. This is the version in "MusicBeePortable_3_4_Update1.zip".
On neither machine did the portable version have the volume slider problem.
On the Win7 laptop both the installed and the portable version mirrored each other in terms of the playlist selected and the track chosen from the playlist, as well as the volume setting. The volume setting remained where it was last placed by either the portable or the installed version. I could open the portable, choose a track, play it, set the volume, stop the track, close the program and return to it to find the volume slider remaining in place when I again chose to play another track: no moving down to zero percent. The same thing happened with the installed version.
The interaction between the portable and the installed versions on the Win10 machine is a different matter. As I said, the portable by itself works perfectly, while the installed version continues to dump the volume slider to zero. The strange thing is that if I open the portable and choose a track, play it, set the volume, and then close it, the volume stays the same. But if I do the same thing but then open the installed version, the installed version comes up with the identical playlist, chosen track, etc, with the volume wherever I left it in the portable version. But when I click on the track to play it, the volume slider immediately goes to zero percent. If I close the installed version at this point, and open the portable version, the portable version slider has also gone down to zero percent.
So what's the difference here? Is it Win 7 vs Win 10? If so, if it's a system problem, why wouldn't the portable have the same problem? I have several audio apps on both the Win 7 and Win 10 machines: MediaMonkey, VLC, Reaper, Audacity and others, and none have any problem with the system audio.
On the Win 7 machine I also have a digital mixing board installed, VoiceMeeter Potato, along with several digital audio cables. I tested the portable MusicBee against the installed version with the system sound as well as with the various digital audio cables routed through VoiceMeeter, and everything performed flawlessly. MusicBee worked great!